Despite the weird name, "deadbugs" work your core unlike any other exercise. Crunches and sit-ups do very little to resist letting your back arch and "extend", which can then contribute to posture problems and low back, knee and hip pain. This is where deadbugs come in. If done correctly, deadbugs strengthen your core while teaching you coordination, and keep your back from extending too much like it would in a crunch or sit-up. To do them, lie on your back and maintain a neutral spine. That means your chin is pulled toward your chest and your lower back remains in constant contact with the ground. Reach both arms and knees toward the ceiling, and slowly extend one arm overhead while simultaneously extending the opposite leg out. Return to the starting position and do the same thing with the other arm and leg.
As you do this, you'll be tempted to lift your back off the ground. You have to fight to keep your back still, and your movements and breathing controlled. Don't rush. Deadbugs are deceptively difficult, and when practised regularly will only make you stronger.
Deadbugs: The What, Why, and How [Tony Gentilecore]